Learning About Light

by: Marty Sagendorf

Light is magic stuff: it has no mass, it comes in many colors, it has energy, it can be emitted and absorbed, but it can’t be saved in a bottle or bucket. Even though we can’t ‘save’ it, we can explore the many ways that light behaves around us. We are told, or we read, about reflection, refraction, and the many other properties of light’s interaction with objects, but until we actually experience these we really don’t fully appreciate ‘the magic of light’.

Let’s start learning about light!

That’s what this Optics Kit, from Educational Innovations, allows us to do – experience light by doing. This kit provides the necessary components to perform extensive investigations – ten are completely detailed – and new ideas for experimentation will naturally develop as optics principles become familiar.

Let’s begin with something we see every day:

REFLECTION

Start by drawing a line along, and one-half inch from, the long side of a piece of 8-1/2”x11” white paper. At the mid-point of this line, draw a perpendicular line extending across the paper. This line represents a ‘NORMAL’ to the mirror’s surface.

Mount the mirror and place it at the junction of the lines as shown.

Now turn the laser pointer ON (by taping the button down), and place it at an angle of about 30 degrees from the NORMAL. Place a piece of card stock in front of the mirror – move the laser to place its beam spot directly above the NORMAL line. Note that in the photo the red laser beam dot appears as a white spot (an artifact of a digital camera). Remove the piece of card stock.

Place the (white) target on as shown and adjust it to place the reflected ray near the center of the target.

Now the rays can be traced (using a card and a pencil) by making a series of dots along both the incident and reflected beam paths. These dot paths should be labeled (‘A’ for example) – the dots will be connected later.

Move the laser to a new position (for example, approximately 60 degrees from the NORMAL). Again, use the piece of card stock to place the laser beam directly above the NORMAL line. Remove the card stock and move the target to a new position showing the reflected beam’s spot. Trace these rays and label each series of dots as ‘B’. More than two angles can be plotted, but two angles suffice to illustrate principle.

Draw ‘best fit’ lines through each of the four sets of dots. Use a protractor to measure the angles of the incident and reflected beam paths – they should be equal for each. Small errors in plotting the points are inevitable. Angular errors of three degrees are quite acceptable.

TOTAL INTERNAL REFLECTION

We need to make two identical sheets (each 8-1/2”x11”) as shown. Add an outline of the prism to each sheet.

REFRACTION will occur at both surfaces if the laser is placed at angles less than 42 degrees from the normal.

The beam paths are plotted as we did with REFLECTION.

Now using the second sheet of paper, we aim the laser at angles greater than 42 degrees. Immediately we see that the beam no longer passes through the prism, but is TOTALLY INTERNALLY REFLECTED by the rear surface of the prism. Again we plot the beam paths with dots and lines.

IMAGE INVERSION – The CONVEX LENS

Cover half the diameter of the LED flashlight lens. Here, for illustrative purposes, I’ve used only one thickness of masking tape – any degree of opacity will work.

Mount the flashlight, lens and target as illustrated. Adjust the distances for ‘clean’ images of the LEDs on the target – these will be roughly a 1:2 ratio.

The image of the flashlight is inverted on the target. Will this still occur if the flashlight is rotated?

DOUBLE-AXIS DIFFRACTION GRATING – ONE COLOR

Here we begin by using the Laser Pointer, a Double-Axis Diffraction Grating and a Large Target.

Here we see the pattern produced – the ‘ORDERS’ of the spectrum (but illustrated by only one color – red).

Now we introduce a second Double-Axis Diffraction Grating. Notice that it performs a double-axis diffraction on each of the original beams.

OTHER EXPLORATIONS

The kit also includes Illustrated & Explained EXPLORATIONS in REFRACTION of CO-PLANAR SURFACES, REFRACTION in SURFACES @ 60 degrees, DISTANT IMAGE PROJECTION, Studies with a Dichroic Filter, and Observations with Multiple Polarizing Filters.

Of course, the breath of experimentation can be enhanced with the inclusion of additional on-hand, or additionally purchased optical components.

AND

Once students discover the fascination of light, they can be challenged to discover other ways illustrating the properties of light. These can be shown as either classroom demonstrations or as the write-ups of new laboratory experiences for other students.

Marty Sagendorf is a retired physicist and teacher; he is a firm believer in the value of hands-on experiences when learning physics. He authored the book Physics Demonstration Apparatus. This amazing book is available from Educational Innovations – it includes ideas and construction details for the creation and use of a wide spectrum of awe-inspiring physics demonstrations and laboratory equipment. Included are 49 detailed sections describing hands-on apparatus illustrating mechanical, electrical, acoustical, thermal, optical, gravitational, and magnetic topics. This book also includes sections on tips and hints, materials sources, and reproducible labels.